Oscillator circuit arrangement

ABSTRACT

An oscillator circuit arrangement comprises an inverter having input and output terminals that are to be connected to a crystal device. An automatic gain control device controls a current source that supplies current to the inverter. First and second diode devices having different orientation are connected between the input and the output of the inverter. The oscillator consumes low power and has a fast recovery time after an electromagnetic interference event. The oscillator can be used in electronic labels.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is the national stage entry of InternationalPatent Application No. PCT/EP2018/083674, filed on Dec. 5, 2018, whichclaims benefit of priority of European Patent Application No.17207021.1, filed on Dec. 13, 2017, all of which are hereby incorporatedby reference in their entirety for all purposes.

BACKGROUND

The present disclosure relates to an oscillator circuit arrangement forlow power applications. Specifically, the present disclosure relates toan oscillator circuit arrangement that comprises an inverter to beconnected to a crystal device and an automatic gain control devicecontrolling the supply current for the inverter. The present disclosurealso relates to the application of the oscillator circuit arrangement inan electronic label.

Oscillators are widely used to provide clock signals to controloperation of electronic devices. Oscillators may be implemented by wayof an integrated circuit on a semiconductor chip with external elementssuch as a crystal to provide a time base and capacitors to provideproper phase shift of the signals.

An oscillator according to the Pierce architecture may be used, forexample, in the field of electronic label or electronic paperapplications where power consumption is crucial. On the other hand,electronic shelf labels may operate in an environment whereelectromagnetic interference (EMI) is present. Modern electronic ballastcircuits of fluorescent lighting systems typically operate in the rangeof about 20 KHz and more so that they may interfere with low poweroscillators operating in a similar frequency range. Because theoscillators for electronic label applications are designed to consume aslittle power as possible, an EMI event can disturb the operation of theoscillator. For example, high frequency ballast circuits generate fasttransients that may couple a strong noise into the oscillator so thatthe oscillator can drift from its ideal operation. The oscillation maystop for a certain amount of time and recover later or, in the worstcase, oscillation may be completely lost and may not recover by itself.The display of information in an electronic shelf label could thereforebe disturbed by EMI or the label could be disturbed such that it stopsoperation.

There is a need to provide an oscillator that is more robust againstelectromagnetic interference and recovers quickly after a strong EMIevent. The oscillator should nevertheless be simple in construction andrequire little area when realized in an integrated circuit. The powerconsumption of the oscillator must be kept at a low level.

SUMMARY

In an embodiment, an oscillator circuit arrangement comprises aninverter having an input terminal and an output terminal to be connectedto a crystal device and having a supply terminal; an automatic gaincontrol device connected to the output terminal of the inverter andconnected to a controllable current source that is connected to thesupply terminal of the inverter; first and second diode devices havingdifferent orientation and connected between the input and the output ofthe inverter.

The oscillator comprises first and second diode devices of differentorientation connected between the input and the output of the inverter.The diode devices may be connected in antiparallel fashion. Whenrealized as diode-connected MOS transistors, the gate and drainterminals are connected together. The gate and drain terminals of one ofthe diodes may be connected to the output of the inverter and the sourceterminal of that diode may be connected to the input of the inverter.The drain and source terminals of the other diode may be connected tothe input of the inverter and the source terminal may be connected tothe output of the inverter. These diodes oppose a big voltage changethat occurs at the input or the output of the oscillator and thereforeprovide a fast stabilization of the output signal voltage if aninterference signal is removed. The diodes provide a forward biasedvoltage to the input and output of the oscillator and clamp theseterminals in a start-up situation. In normal, steady state mode of theoscillator, the diodes are substantially back-biased. The diodes drawonly a current in the exceptional situation of start-up after an EMIevent and are substantially switched off during normal operation.

An electronic shelf label that incorporates the oscillator may haveenough refresh delay so that the oscillator can recover its oscillationbefore the next display refresh, even after a complete loss ofoscillation in the case of an EMI event so that the loss of oscillationwill not be recognized by a person looking onto the display.

According to an embodiment, the oscillator may comprise a controllableresistor that is connected between the input and the output of theinverter. The controllable resistor has a higher resistance in thetransient time, for example, during startup and a lower resistance innormal mode so that the inverter has a higher gain during the ramp-upphase. A separate current path that controls the controllable resistorincludes a controllable current source that is controlled by theautomatic gain control (AGC) device. The separate current path mayinclude a resistive element that is connected between the controllablecurrent source and ground potential. Specifically, the resistive elementmay comprise the series connection of a first and a second diode device.The separate current path therefore resembles the conditions in thecurrent path of the inverter.

According to yet another embodiment of the present disclosure, a thirddiode device may be provided that is connected between the first andsecond diode devices of the resistive element of the separate currentpath and the output of the inverter. The third diode device has asimilar effect to the first and second oppositely connected diodedevices in that it opposes a big voltage change in the output node andprovides a fast stabilization of the output signal in the case of an EMIevent.

The oscillator circuit may be realized in CMOS technology so that alldiode devices are realized by MOS transistors of which the drain andgate terminals are connected together. The oppositely connected firstand second diode devices and the third diode device may be n-channel MOStransistors. The controllable resistor that is connected between inputand output of the inverter may also be a n-channel MOS transistor. Thecontrollable current sources of the inverter and of the separate currentpath may be p-channel MOS transistors whose gate terminal is connectedto the output of the AGC device. The source terminals of the p-channelMOS transistors are connected to a positive supply potential. Therealization of the circuit with CMOS technology allows a design thatrequires only little supply power. The additional diodes that enable afast start-up time are only active during the transient phase and areoff during normal operation so that they require no additional powerduring normal operation.

The automatic gain control (AGC) device includes circuitry thatgenerates a high gain during the startup or transient operational phaseof the oscillator and has a low gain in the normal or steady stateoperational phase. The AGC device controls the conductivity state of thecontrollable current sources and causes them to supply more currentduring the transient phase and only as low as possible current duringthe steady state phase.

According to another aspect of the present disclosure, an electroniclabel device comprises a display device and the oscillator circuitarrangement as mentioned above. The oscillator circuit arrangementprovides a clock signal to control display of information on the displaydevice.

The display screen of the electronic label requires a refresh operationthat is controlled by the oscillator. Because the oscillator circuitarrangement has an accelerated start-up time, the oscillator may recoveroscillation even after a loss of oscillation in response to an EMI eventwithin the refresh interval before the next refresh operation isnecessary. A person who watches the display will not recognize aninterruption. All this is achieved with only very few extra elements inthe integrated circuit while still fulfilling the requirement of verylow power consumption.

Additional features and advantages will be set forth in the detaileddescription which follows and in part will be readily apparent to thoseskilled in the art from the description or recognised by practising theembodiments as described in the written description and claims hereof aswell as the appended drawings.

The accompanying drawings are included to provide a furtherunderstanding and are incorporated in and constitute a part of thedisclosure. The drawings illustrate one or more embodiments, andtogether with the description serve to explain principles and operationof the various embodiments. The same elements in different Figures aredenoted by the same reference signs.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 shows a principle circuit representation of a common Pierceoscillator;

FIG. 2 shows a detailed circuit diagram of an oscillator circuitarrangement according to this disclosure;

FIG. 3 shows a principle circuit diagram of an automatic gain controlcircuit; and

FIG. 4 shows a principle diagram of an electronic label incorporating anoscillator according to the present disclosure.

DETAILED DESCRIPTION

It is to be understood that both the foregoing general description andthe following detailed description are merely exemplary and are intendedto provide an overview or framework to understand the nature andcharacter of the claims. The present disclosure will now be describedmore fully herein with reference to the accompanying drawings showingembodiments of the disclosure. The disclosure may, however, be embodiedin many different forms and should not be construed as limited to theembodiments set forth herein. Rather, these embodiments are provided sothat the disclosure will fully convey the scope of the disclosure tothose skilled in the art. The drawings are not necessarily drawn toscale but are configured to clearly illustrate the disclosure. Sameelements in different drawings are denoted by the same referencenumerals.

FIG. 1 shows the principle diagram of a common oscillator. An inverterINV is disposed on an integrated circuit IC. The input and the output ofthe inverter are connected to an external quartz crystal XC that forms atime basis for the oscillation of the circuit. The input and the outputof the inverter are furthermore coupled to ground via two capacitors C1,C2. The crystal and the capacitors are external to the integratedcircuit IC while the inverter and other components are integrated on theintegrated circuit IC. The output signal of the inverter INV ismonitored by an automatic gain control device AGC that controls theamplitude of the oscillation by a feedback loop. The AGC has a highamplification factor g_(m) during a transitional phase of operation ofthe oscillator. During the normal or steady state phase of theoscillator, the amplification factor g_(m) is low and the loop gain isclose to unity. The architecture of the oscillator as shown in FIG. 1 isthe so-called Pierce oscillator circuit.

FIG. 2 shows a detailed circuit diagram of an oscillator according tothe Pierce architecture that includes improvements to resolve theobjects explained above. The oscillator circuit comprises an invertercomposed of p-channel MOS transistor MP1 and n-channel MOS transistorMN1 having series-connected drain-source paths. The output XOUT of theinverter is sensed by an automatic gain control circuit AGC thatcontrols a controllable current source realized by p-channel MOStransistor MP2. The drain terminal of transistor MP2 is connected to thesource terminal of transistor MP1 of the inverter which is the supplyterminal of the inverter. The external crystal XC and the capacitors C1,C2 (not shown in FIG. 2) are to be connected between the input andoutput terminals XIN, XOUT of the inverter. Depending on the amplitudeof the output signal at XOUT, the AGC circuit controls the current to bedelivered by controllable current source MP2.

A separate current path comprises another controllable current sourcerealized by p-channel MOS transistor MP3. The gate of transistor MP3 isalso connected to the output of the AGC circuit and is connectedparallel to the gate of controllable current source MP2. The drain oftransistor MP3 is connected through a resistive element to groundpotential GND. The resistive element comprises the series connection oftwo MOS diode devices MN3, MN2. The current through the separate currentpath MP3, MN3, MN2 has the same control and the same transient behaviouras the current through the inverter current paths MP2, MP1, MN1.

A controllable resistor in the form of n-channel MOS transistor MN4 isconnected between the output and the input of the inverter. The drainterminal MN4 is connected to the output terminal XOUT, and the sourceterminal of transistor MN4 is connected to the input terminal XIN. Thegate terminal of transistor MN4 is connected to the node between thedrain terminal of diode MN3 and the current source MP3. If the currentthrough the separate current path MP3, MN3, MN2 is high, thecontrollable resistor MN4 has a low resistance. This is the case in atransitional phase or during the ramp-up phase of the oscillator. If thecurrent through the separate current path is low, the resistance oftransistor MN4 is high, which is the case during normal, steady stateoperation of the oscillator.

In accordance with an embodiment of this disclosure, first and seconddiode devices MND2, MND3 are provided that have opposite orientation andthat are connected between the output terminal XOUT and the inputterminal XIN of the inverter. Both diode devices are realized asn-channel MOS transistors of which the gate and drain terminals areconnected together. The gate and drain terminals of the first diodedevice MND2 are connected to the output terminal XIN, and the sourceterminal of diode device MND2 is connected to output terminal XOUT. Thegate and drain terminals of the second diode MND3 are connected tooutput terminal XOUT, and the source terminal of diode device MND3 isconnected to the input terminal XIN.

If a strong EMI event takes place, the oscillation of the circuit islikely to be disturbed. The EMI noise may impress the noise frequency onthe oscillator circuit. When a strong interference signal due to aballasted lighting device such as a fluorescent lamp is injected intothe terminals XOUT and XIN or both, the AGC will reduce the drive levelthrough reducing the amplification factor g_(m) by reducing the currentthrough the controllable current sources MP2, MP3. In the case of apersistent interference signal with long duration, the factor g_(m) maybe reduced to the point that the oscillator stops its oscillation. TheAGC reacts in response to the noise impulse such that it reduces thecurrent through the controllable current source MP2 and thereforethrough the inverter so that the oscillation may stop. In this case, atleast one of the first and second MOS diodes MND2, MND3 will beconductive and will cause a current flowing between the input and theoutput XIN, XOUT of the inverter so that the oscillator quickly restartsagain. During normal operation, however, the diode devices MND2, MND3are switched off.

In yet another embodiment of the present disclosure, a third diodedevice MND1 is provided which is realized as an re-channel MOStransistor of which the gate and drain terminals are connected together.The gate and drain terminals of the third diode MND1 are connected tothe node between the two series-connected diode devices MN2, MN3. Thesource of the third diode device MND1 is connected to the outputterminal XOUT. During the startup phase, the signal at the outputterminal XOUT will be limited to two forward diode voltages above groundpotential GND through diode transistors MN2, MND1. It is to be notedthat the terminal XOUT is shown twofold in the circuit diagram tosimplify the structure of the circuit. In fact, the terminal XOUT existsonly once in the circuit. Instead, both terminals labelled with XOUTcould be connected by a wire.

During operation, the diode-connected transistors MND1, MND2, MND3monitor the voltage at the terminals XOUT and XIN, and the voltage ofdiode MN2 is common to both terminals XIN, XOUT through the MOS resistorMN4, which is substantively conductive. These diodes oppose a bigvoltage change in these nodes and provide a fast stabilization of thesignal at output terminal XOUT, once the electromagnetic interferenceevent is removed. The amplitude of the oscillation is regulated, sincethe lower extremes of the sine wave of the oscillation are restrained tobe equal to the voltage at the input terminal XIN minus the forwardvoltage of the diode MND2.

In the case that the voltage at terminal XIN is very low, the diode MND3will be forward biased so that a current will flow from the output XOUTto the input XIN. Furthermore, if the output voltage at terminal XOUT isvery low, the diode MND2 will be forward biased to provide a currentinto output terminal XOUT. The diode devices MND1, MND2, MND3 accordingto the present disclosure will only be active if the oscillation is notin the normal, steady state so that they do not consume power in normaloperation mode. On the other hand, the AGC controls the controllablecurrent sources MP2, MP3 to a low level in normal operation mode. In atransitional phase, at least one of the diodes MND1, MND2, MND3 will beforward biased so that the oscillation has a chance to restart and theAGC increases the current through controllable current sources MP2, MP3.The circuit can be designed for very low power operation in the steadystate mode, while it has provisions to enhance the restart of theoperation in the case of an EMI event.

FIG. 3 shows a principle diagram of the AGC circuit. A peak detector Pis connected to the output terminal XOUT. The output signal of the peakdetector is compared to a reference voltage VREF. The signal at theoutput of the comparator C is representative of the amplification factorg_(m) and controls the current through the oscillator O. Otherrealizations of the AGC circuit apparent to a skilled person are alsouseful.

When the oscillator has not yet started up, the AGC behaves as a biascurrent reference circuit to provide the necessary start-up current forthe Pierce core. At the start-up the g_(m) is set to be higher than therequired critical point to accelerate oscillation build-up. As theoscillation amplitude builds up, the reference current sourced by MP2and controlled by the regulator is reduced until the AC voltageamplitude reaches a critical point where the loop gain approaches unity.At the stable oscillation condition, the oscillator is operating at theminimum current required to address the low power requirement.

FIG. 4 describes a principle circuit diagram of an electronic equipmentincorporating the oscillator of this disclosure. An electronic label fora shelf in a supermarket such as an electronic shelf label ESL is shownin FIG. 4. The electronic shelf label comprises a display DSP on whichinformation is displayed, such as price information, price per kilograminformation, quantity information, etc. The display may be a liquidcrystal display (LCD) or may be of a reflective type. Usually, thedisplay requires a refresh operation and is operated in a line-by-linesequence. The display is controlled by a clock time basis generated bythe oscillator OSC. The time basis is used, for example, to control theline-by-line refresh operation of the display.

The information to be displayed is provided by a processor PROC. Theprocessor is controlled by a clock signal also provided by theoscillator OSC.

The electronic shelf label is usually attached to the front edge of ashelf, often in the vicinity of a fluorescent lighting system that mayalso be attached to the shelf and that may become a great source of anEMI event to the neighboring shelf labels. Modern fluorescent lamps haveelectronic ballast circuits that typically operate in the range of about20 kHz to about 120 kHz and may cause interference with the electronicshelf labels. An EMI event can couple such a strong noise into theoscillator circuit including electrically fast transients or highfrequency interference signals so that the oscillator may drift from itsideal operation or may even end up in loss of oscillation. Conventionaloscillators may risk ending in a situation where they never can recoveroscillation.

The oscillator according to the present disclosure has first, second andthird diode devices that provide for a fast recovery of oscillationafter a stop of oscillation. With a suitable design of the device, therecovery time can be so quick that it is stable again before the end ofthe refresh interval so that the display of information will be not bedisturbed.

1. An oscillator circuit arrangement, comprising: an inverter having aninput terminal and an output terminal to be connected to a crystaldevice and having a supply terminal; an automatic gain control deviceconnected to the output terminal of the inverter and connected to acontrollable current source that is connected to the supply terminal ofthe inverter; and first and second diode devices having differentorientation and connected between the input and the output of theinverter.
 2. The oscillator circuit arrangement according to claim 1,further comprising a controllable resistor connected between the inputand output terminals of the inverter and a current path comprisinganother controllable current source controlled by the output of theautomatic gain control device, the current path controlling thecontrollable resistor.
 3. The oscillator circuit arrangement accordingto claim 2, wherein the current path includes at least one resistiveelement through which the controllable current source of the currentpath is connected to a terminal for a ground potential.
 4. Theoscillator circuit arrangement according to claim 3, wherein the atleast one resistive element of the current path comprises a seriesconnection of a first and a second diode device.
 5. The oscillatorcircuit arrangement according to claim 4, further comprising a thirddiode device connected between one of the first and second diode devicesof the resistive element of the current path and the output terminal ofthe inverter.
 6. The oscillator circuit arrangement according to claim5, wherein the third diode device comprises a MOS transistor of whichthe gate and drain terminals are connected with each other.
 7. Theoscillator circuit arrangement according to claim 2, wherein the firstand second diode devices each comprise a MOS transistor of which thegate and drain terminals are connected with each other.
 8. Theoscillator circuit arrangement according to claim 4, wherein the firstand second diode devices of the resistive element each comprise a MOStransistor of which the gate and drain terminals are connected with eachother.
 9. The oscillator circuit arrangement according to claim 6,wherein the MOS transistors of the first, second and third diode devicesand the controllable resistor, in each case, are n-channel-MOStransistor.
 10. The oscillator circuit arrangement according to claim 3,wherein the controllable current sources, in each case, are ap-channel-MOS transistor whose gate terminal is connected to the outputof the automatic gain control device.
 11. The oscillator circuitarrangement according to claim 1, wherein the first diode device has agate terminal and a drain terminal that are connected with each otherand are connected to the input of the inverter and has a source terminalthat is connected to the output of the inverter.
 12. The oscillatorcircuit arrangement according to claim 1, wherein the second diodedevice has a gate terminal and a drain terminal that are connected witheach other and are connected to the output of the inverter and has asource terminal that is connected to the input of the inverter.
 13. Theoscillator circuit arrangement according to claim 1, wherein theautomatic gain control device generates an output signal that has a highgain close to the startup operational phase of the oscillator and has alow gain in the steady state operational phase of the oscillator. 14.The oscillator circuit arrangement according to claim 1, wherein theoscillator circuit comprises a terminal for a supply potential, whereinthe controllable current source is connected to the terminal for asupply potential and the supply terminal of the inverter, thecontrollable current source having a control terminal, wherein theautomatic gain control device has an input terminal, connected to theoutput terminal of the inverter and an output terminal connected to thecontrol terminal of the controllable current source, wherein the firstand second diode devices each comprise a MOS transistor having gate,drain and source terminals of which gate and drain terminals areconnected with each other, wherein the gate and drain terminals of thefirst diode device are connected to the input of the inverter and thesource terminal of the first diode device is connected to the output ofthe inverter, and wherein the gate and drain terminals of the seconddiode device are connected to the output of the inverter and the sourceterminal of the second diode device is connected to the input of theinverter.
 15. An electronic label device comprising a display device andthe oscillator circuit arrangement comprising: an inverter having aninput terminal and an output terminal to be connected to a crystaldevice and having a supply terminal, an automatic gain control deviceconnected to the output terminal of the inverter and connected to acontrollable current source that is connected to the supply terminal ofthe inverter; and first and second diode devices having differentorientation and connected between the input and the output of theinverter, wherein the oscillator circuit arrangement provides a clocksignal to control display of information on the display device.